Integrated circuits and packaging substrates with cavities, and attachment methods including insertion of protruding contact pads into cavities

ABSTRACT

A packaging substrate ( 310 ) includes a semiconductor interposer ( 120 ) and at least one other intermediate substrate ( 110 ), e.g. a BT substrate. The semiconductor interposer has first contact pads ( 136 C) attachable to dies ( 124 ) above the interposer, and second contact pads ( 340 ) attachable to circuitry below the interposer. Through vias ( 330 ) are made in the semiconductor substrate ( 140 ) of the interposer ( 120 ). Conductive paths going through the through vias connect the first contact pads ( 136 C) to the second contact pads ( 340 ). The second contact pads ( 340 ) protrude on the bottom surface of the interposer. These protruding contact pads ( 340 ) are inserted into vias ( 920 ) formed in the top surface of the BT substrate. The vias provide a strong mechanical connection and facilitate the interposer handling, especially if the interposer is thin. In some embodiments, an interposer or a die ( 124.1 ) has vias in the top surface. Protruding contact pads ( 340.1, 340.2 ) of another die ( 124.1, 124.2 ) are inserted into these vias to provide a strong connection.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/739,788 filed on Dec. 17, 2003 by S. Savastiouk et al., entitled“INTEGRATED CIRCUITS AND PACKAGING SUBSTRATES WITH CAVITIES, ANDATTACHMENT METHODS INCLUDING INSERTION OF PROTRUDING CONTACT PADS INTOCAVITIES”, incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to attachment of integrated circuits toother integrated circuits and/or intermediate substrates.

Integrated circuit dies (“chips”) can be attached to a lead frame andthen packaged in a ceramic or plastic carrier. The leads of the leadframe can then be soldered to a printed circuit board (PCB).Alternatively, the chip can be soldered directly to the PCB (“flip chip”packaging). The flip chip packaging reduces the package size andshortens the electrical connections between the die and the PCB, but theflip chip packaging is vulnerable to solder failures caused by thermalexpansion and contraction. The solder failures are due to thedifferences in the coefficient of thermal expansion (CTE) between thedie and the PCB.

The CTE mismatch has been addressed by providing an intermediatesubstrate between the die and the PCB, with an intermediate CTE. Forexample, in a ball grid array (BGA) package shown in FIG. 1, die (“IC”)124 is flip-chip attached, with solder 126, to the intermediatesubstrate 110 (“BGA substrate”), and BGA substrate 110 is soldered toPCB 130 (with solder 134). BGA substrate 110 provides interconnect lines(not shown) between die 124 and PCB 130. A silicon die 124 may have aCTE of about 2.7 ppm/° C. (parts per million per degree Centigrade); aPCB made of FR4 can have a CTE of about 20 ppm/° C.; a BGA substratemade from BT (bis-maleimide triazine) has a CTE of about 16 ppm/° C.,and a BGA substrate made from ceramic has a CTE of about 9 ppm/° C.

In addition to reducing the thermal stresses, the intermediate substrate110 may allow a smaller die size by allowing the die 124 to have smallercontact pads with a reduced pitch. The minimum size and pitch of thedie's contact pads is limited by the size and pitch of the contact padson the substrate to which the die is attached. For example, if the dieis flip-chip bonded to a BT substrate, the size and pitch of the die'scontact pads can be smaller than if the die is attached to an FR4substrate (PCB).

Intermediate substrate 110 may also reduce the PCB area taken by the diebecause the intermediate substrate may redistribute the die's contactpads. The position of the die's contact pads is restricted by the die'scircuitry. The BGA substrate's contact pads that are bonded to the PCBare not restricted by the die's circuitry. For example, the die may havecontact pads only on the periphery, but the BGA substrate's contact padsattached to the PCB may be evenly distributed over the BGA area.

Further, if multiple dies 124 are mounted on a single intermediatesubstrate 110, the dies can be interconnected by interconnects in theintermediate substrate without using the PCB routing resources. Thisleads not only to saving the PCB area but also to shorterinterconnections between the dies and hence to a better electricalperformance (higher speed and lower power consumption, inductance andcapacitance).

FIG. 2 illustrates another package with two intermediate substrates 110,120 between dies (ICs) 124 and PCB 130. Intermediate substrate 110 is aBT substrate, soldered to the underlying PCB 130 with solder balls 134.Intermediate substrate 120 is a silicon interposer attached to the topsurface of BT substrate 110 by an adhesive (not shown). Siliconinterposer 120 includes metal layers 136 formed over silicon substrate140 and separated by dielectric layers 144. Dies 124 are attached tointerposer 120 with their contact pads facing up. The dies' contact padsare wire bonded to contact pads 136C.1 provided by metal layers 136. Thewire bonding is done with bond wires 150. Contact pads 136C.2 on top ofthe interposer are wire bonded to contact pads 360 on top of BTsubstrate 110 using bond wires 160. Interconnect lines made from layers136 connect the contact pads 136C.1 to the contact pads 136C.2.

Metal layers 136 provide interconnects between the dies 124. Theinterconnects can be manufactured on silicon interposer 120 with ahigher density and higher electrical performance than on BT substrate110. There is no CTE mismatch between silicon substrate 120 and silicondies 124.

We will use the term “packaging substrate” for each of substrates 110,140, and for a structure consisting of the substrates 110 and 140attached to each other. It is desirable to provide a reliable attachmentbetween the packaging substrates 110, 140. The attachment should bemechanically strong. The attachment methods should minimize any breakageof the interposer 120, especially if the interposer is thin.

It is also desirable to provide a strong, reliable attachment ofintegrated circuits to each other and to packaging substrates.

SUMMARY

This section summarizes some features of the invention. Other featuresare described in the subsequent sections. The invention is defined bythe appended claims which are incorporated into this section byreference.

In some embodiments of the present invention, a packaging substrate isprovided which, like the packaging substrate of FIG. 2, includes asilicon interposer and a BT substrate. However, the silicon interposerhas contact pads both on the top and the bottom, and has through-siliconvias made in the silicon substrate of the interposer. Conductive pathsgoing through the through-silicon vias connect the contact pads on thetop of the interposer to the contact pads on the bottom. The contactpads protrude on the bottom surface of the interposer. The protrudingcontact pads are inserted into vias formed in the top surface of the BTsubstrate. The vias facilitate the interposer handling, especially ifthe interposer is thin. The vias also increase the mechanical strengthand thermal-stress reliability of the structure.

Silicon interposers with through-silicon vias have been described inU.S. Pat. No. 6,322,903, incorporated herein by reference, but not in apackaging substrate having two or more intermediate substrates as insome embodiments of the present invention. The packaging substratesaccording to some embodiments of the present invention provide amanufacturing challenge if the silicon interposer is thin. Thininterposers are desirable to reduce the package size and improve theelectrical characteristics (by shortening the conductive paths throughthe interposer). Also, in some embodiments, it is easier to manufacturethe through-silicon vias if the interposer is thin. However, thininterposers are fragile, can be warped, and their heat dissipationcapabilities are poor, so the interposer handling is complicated. InU.S. Pat. No. 6,322,903, at least in some embodiments, the interposer isthinned only after attachment to a die. However, in a packagingsubstrate, the interposer may have to be thinned to its final thicknessbefore the die attachment. In some embodiments, the interposer isthinned before attachment to the BT substrate. The semiconductorsubstrate of the interposer can be quite thin, e.g. 100 μm or thinner.The semiconductor substrate and the interposer may have substantiallyplanar top and bottom surfaces, as opposed to interposers with cavitieslarge enough to contain a die, with the cavities' sidewalls beingthicker than the rest of the interposer to increase the interposer'smechanical strength (see U.S. patent application Ser. No. 09/952,263filed Sep. 13, 2001 by Halahan et al., incorporated herein byreference). The term “substantially planar” indicates that anynon-planarity of the semiconductor substrate or the interposer is sominor as to have no significant effect on the mechanical strength of thestructure.

Some aspects of the present invention relate to a manufacturing process,and to a BT substrate, that simplifies the handling of thin siliconinterposers.

The via structures can also be used to attach the integrated circuits toeach other and to packaging substrates. For example, in someembodiments, an integrated circuit die has contact pads protruding onits bottom surface. These contact pads can be inserted into vias formedin the top surface of an interposer or another die to increase thestrength of the structure.

The invention is not limited to the embodiments discussed in thissection. The invention is not limited to thin interposers, and furtheris applicable to non-silicon semiconductor interposers attached tonon-BT intermediate substrates. Other features and advantages of theinvention are described below. The invention is defined by the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show vertical cross sections of integrated circuitpackaging structures according to prior art.

FIGS. 3-13 show vertical cross sections of integrated circuit packagingstructures according to some embodiments of the present invention.

FIG. 14 is a flowchart of an integrated circuit packaging processaccording to one embodiment of the present invention.

FIG. 15 shows a vertical cross section of an integrated circuitpackaging structure according to one embodiment of the presentinvention.

DESCRIPTION OF SOME EMBODIMENTS

The embodiments described in this section illustrate but do not limitthe invention. The invention is not limited to particular materials,process steps, or dimensions. The invention is defined by the appendedclaims.

FIG. 3 illustrates an integrated circuit packaging substrate 310 havingtwo intermediate integrated circuit packaging substrates 110, 120.Substrate 120 is a silicon interposer attached to BT substrate 110. Dies124 and PCB 130 will be attached later.

Silicon interposer 120 includes metal layers 136 formed over siliconsubstrate 140. Substrate 140 has substantially planar top and bottomsurfaces, and is quite thin. In some embodiments, the planarity ofsubstrate 140 is suitable for fine geometry photolithography (finer thanpossible with BT and FR4 substrates). The thickness of substrate 140 canbe 100 μm or less (50 μm to 35 μm thickness values believed to beachievable, and smaller values may be possible). Layers 136 provideinterconnect lines and may also provide power and ground planes,resistors, inductors, capacitor plates for decoupling capacitors andother capacitor types, and possibly other elements, known or to beinvented. Layers 136 can be separated from each other, and from thesubstrate, by dielectric layers 144. Layers 136 contact each other andthe silicon substrate through openings in the dielectric layers. Layers136 can also be formed directly on the silicon substrate if desired.Layers 136 provide contact pads 136C at the top surface of theinterposer. The contact pads are available for flip-chip attachment todies 124.

Silicon substrate 140 includes metalized through-silicon vias 330 thatpass between the top and bottom surfaces of substrate 140. Conductivepaths are provided from contact pads 136C at the top of the interposerto contact pads 340 at the bottom of the interposer through the vias330. Contact pads 340 are attached to contact pads 350 at the topsurface of BT substrate 110.

Interconnects (not shown) in BT substrate 110 connect the contact pads350 to contact pads 360 at the bottom surface of substrate 110. Solderballs 134 are formed on pads 360 by conventional techniques forattachment to PCB 130.

The size and spacing (pitch) of contact pads 136C on interposer 120matches the size and the pitch of the contact pads on dies 124. If dies124 are silicon integrated circuits, their CTE matches the CTE of theinterposer, so the pitch of contact pads 136C can be small because thelow thermal stresses at the interface between the dies and theinterposer make it unnecessary to use large solder balls 370. Thecontact pads 340 on the bottom of the interposer match the top contactpads 350 of BT substrate 110. For some fabrication technologies, theminimum dimensions are as shown in the following table. The dimensionscan typically be reduced if more expensive technologies are used.Minimum Contacts pitch Solder ball diameter Solder ball height Contactpads 136C  125 μm 60 μm (solder balls  50 μm 370 on IC 124) Contact pads340,  254 μm 350 Contact pads 360 1.27 mm 0.5 mm (solder balls 0.4 mm134)

To facilitate the interposer handling, the metal contact pads 340 areformed to protrude out of vias 330. The protruding contact pads 340 areinserted into cavities in BT substrate 110, as explained in more detailbelow. The invention is not limited to the protruding contact pads orthe cavities however.

Silicon interposer 120 can be manufactured using conventionaltechniques. See e.g. the aforementioned U.S. Pat. No. 6,322,903. Othertechniques are described in U.S. patent application Ser. No. 10/410,929filed on Apr. 9, 2003 by P. Halahan et al., entitled “Electroplating andelectroless plating of conductive materials into openings, andstructures obtained thereby”, incorporated herein by reference. Stillother techniques can possible be used, whether known or to be invented.An exemplary manufacturing process is as follows. Vias 330 (FIG. 4) areetched in the top surface of silicon substrate 140 (e.g. monocrystallinesilicon) by DRIE (deep reactive ion etching) to an exemplary depthHv=150 μm. (The dimensions, etching processes, and other particulars areexemplary and not limiting.) The via diameter Dv is 25 μm to 100 μm. Thevia diameter DV is one of the parameters defining the diameter ofcontact pads 340 (FIG. 3), and DV is chosen large enough to provide thenecessary mechanical strength for the protruding contact pads. Exemplarydimensions below will be given for Dv=65 μm. Silicon dioxide layer 410is thermally grown on the wafer to a thickness of about 1 μm. A largerthickness can also be used to reduce the capacitance between substrate140 and the metal features that will be fabricated in vias 330. Barrierlayer 420 of titanium-tungsten (TiW) is sputtered on oxide 410 to athickness of 0.2 μm. A seed copper (Cu) layer 430.1 is sputtered on thewafer to a thickness sufficient to ensure a continuous copper coveragein the via. Thicknesses of 0.5 μm to 2 μm are believed to be adequate,depending on the sputter technology. A dry photoresist film 440 isdeposited on the wafer and patterned to expose the vias 330.

Optionally, gold (Au) layer 444 and nickel (Ni) layer 448 areelectroplated, in that order, to an exemplary thickness of 0.2 μm and1.0 μm respectively.

Copper 430.2 is electroplated on nickel 448 to fill the vias 330 andpossibly protrude out of the vias. In the electroplating of layers 444,448, 430.2, the cathode terminal (not shown) of the power source isplaced at the periphery of wafer 140 in physical contact with seed layer430.1.

Optionally, nickel (Ni) layer 450 is electroplated on the top surface ofcopper layer 430.2 to an exemplary thickness of 0.5 μm.

Resist 440 is removed (FIG. 5). A wet copper etch removes the exposedportions of seed copper 430.1, with nickel 450 acting as a mask. Nickel450 protects copper 430.2 in vias 330. Copper 430.2, 430.1 can be etchedlaterally during the wet etch, but the lateral etch does not remove thecopper over the vias 330 because the copper extends laterally beyond thevia edges. In those embodiments in which the nickel 450 is omitted, thecopper etch may reduced the thickness of copper 430.2, but this isacceptable if the copper protrusions above the vias are sufficientlythick. In either case, it is desirable for the top surface of copper430.2 to be at or above the top surface of oxide 410 after the copperetch.

Then a CMP step (chemical mechanical polishing) is performed to removecopper 430.2, nickel 448, gold 444, and TiW 420 off the top surface ofsubstrate 140 (FIG. 6). The CMP stops on oxide 410. The structure has aplanar top surface.

In an alternative embodiment, the wet etch of copper 430.1 is omitted,and copper 430.1 is removed by the CMP step. The separate wet etch ofcopper 430.1 may be desirable however because it may shorten the moreexpensive CMP step, thus reducing the total manufacturing cost.

Oxide 410 can be patterned if desired. Metal layers 136 (FIG. 7) anddielectric layers 144 are deposited on the interposer wafer andpatterned to provide interconnects and, possibly, other elements asdescribed above. In some embodiments, metal 136 is copper and dielectric144 is polyimide, but other materials can also be used. Some or all ofdielectric layers 144 can be silicon dioxide, photosensitivebenzocyclobutene (BCB), polybenzoxazole (PBO), or other materials. For acapacitor, a high dielectric constant material (such as Ta₂O₅) can beused. Aluminum, conductive polysilicon, and other materials can be usedas layers 136. Solder wettable materials (e.g. Ni or Au) can be platedon contact pads 136C if desired.

Then the interposer wafer is thinned from the bottom to expose the gold444. See FIG. 8. The exposed metal provides the contact pads 340 (FIG.3) that will be soldered to BT substrate 110. The wafer thinning can beperformed with any of the techniques described in the aforementionedU.S. Pat. No. 6,322,903 and U.S. patent application Ser. No. 10/410,929.See also U.S. Pat. No. 6,498,381 issued on Dec. 24, 2002 to Halahan etal. and incorporated herein by reference. In one embodiment, the waferthinning includes a CF₄ plasma etch at atmospheric pressure. The plasmaetch exposes the oxide 410 and then etches the silicon 140, oxide 410and TiW 420 selectively to copper 430.1. (Copper 430.1 is etched lateras explained below.) The plasma etch etches silicon 140 faster thanoxide 410, so the oxide protrudes out of the silicon on the bottomsurface of the wafer after the etch. In one embodiment, the finalthickness “Tsif” (marked in FIG. 8) of silicon substrate 140 is 100 μm,and it can be smaller (e.g. 35 μm). Oxide 410 and TiW 420 form 5 μmprotrusions around the copper 430.1 below the silicon surface.

The plasma etch forms copper oxide (not shown) on the exposed portionsof copper 430.1. The copper oxide and the copper 430.1 are etched by awet etch to expose gold 444. The gold provides a solderable oxide-freesurface. Nickel 448 will prevent copper diffusion from layer 430.2 intothe solder. The copper diffusion may be undesirable because it increasesthe solder melting temperature. In other embodiments, the copperdiffusion is desirable to achieve a certain solder hierarchy (thehierarchy of the melting temperatures of different solders) as explainedbelow. In such embodiments, the etch of copper 430.1 can be omitted.

As stated above, gold 444 can be omitted. The etch of copper 430.1 willthen expose nickel 448.

In some embodiments, the copper 430.1 is not etched away. The copperoxide (not shown) on copper 430.1 can be removed by a wet etch. Thecopper oxide can also be removed by a solder flux during soldering ofthe interposer wafer to BT substrate 110 (the soldering operation isdescribed below). Layers 444, 448 can be omitted.

Metal contact pads 340 are metal protrusions formed by the metal layers430.2, 448, 444, 430.1, 420 below the bottom surface of silicon 140. Insome embodiments, the height Hd of metal contact pads 340 is 50 μm.

A dielectric layer (not shown) can optionally be formed on the bottomsurface of the interposer to cover the silicon 140 but not the metalcontact pads 340. The dielectric can be formed without photolithography.See the aforementioned U.S. Pat. Nos. 6,322,903 and 6,498,381 and U.S.patent application Ser. No. 10/410,929.

The interposer wafer can be diced if desired. The dicing can beperformed at the same time as the interposer wafer thinning if vias wereformed along the dicing lines (scribe lines) simultaneously with vias330 at the stage of FIG. 4. See U.S. Pat. No. 6,498,074 issued Dec. 24,2002 to Siniaguine et al., entitled “THINNING AND DICING OFSEMICONDUCTOR WAFERS . . . ”, incorporated herein by reference.

In some embodiments, the interposer wafer is not diced. ICs 124 will beattached to the wafer.

In some embodiments, metal 430.2 does not fill the through-silicon vias.Metal 430.2 is a thin film deposited over the via sidewalls, and it canbe part of a layer 136. See the aforementioned U.S. Pat. No. 6,498,381.Also, in some embodiments the contact pads 340 do not protrude out ofthe bottom surface of the interposer.

Interposer 120 (diced or undiced) can be attached to a conventional BTsubstrate 110 with solder, conductive epoxy, anisotropic adhesive,thermocompression, or possibly by other techniques, known or to beinvented. In some embodiments, however, specially processed BTsubstrates are used to minimize the interposer handling. The interposerhandling should preferably be minimized if the interposer is thin. Theinterposer's silicon substrate 140 can be 100 μm or thinner, theinterposer can be fragile, and its heat dissipation capability can below. Also, the interposer can be warped. Further, some conventionalsoldering techniques, e.g. the techniques that involve electroplating ofsolder and under-ball metallurgy layers on contact pads 340, may requirephotolithography on the bottom surface of the interposer. The use ofphotolithography is undesirable because of possible wafer damage andmask misalignment. The use of a conventional BT substrate can also bedifficult due to a possibly non-uniform height of protruding contactpads 340. Those contact pads 340 that have a smaller height may beunable to reach the BT substrate contact pads 350 (FIG. 3). Therefore, aspecially processed BT substrate 110 is used in some embodiments, asshown in FIGS. 9 and 10.

BT substrate 110 of FIGS. 9 and 10 is formed from one or more BT layerslaminated in a conventional manner. Three layers 110.1, 110.2, 110.3 areshown, but any number of layers can be present. Thin film metal layers910 (e.g. copper) are formed on BT layers 110.i (i=1, 2, 3) and on thebottom side of layer 110.1 in a conventional manner to provide signalrouting paths and ground and power planes. Layers 910 are interconnectedthrough vias in the BT layers 110.i (i.e. 110.1, 110.2, 110.3) usingknown techniques to provide conductive paths between contact pads 350and contact pads 360. The bottom metal layer 910 provides contact pads360 (FIG. 3) at the bottom surface of BT substrate 110.

The difference between the BT substrate 110 of FIG. 9 and a conventionalBT substrate is that the top contact pads 350, and the top metal layer910, are formed below the top BT layer 110.3. In FIG. 9, the top contactpads 350 and the top metal 910 are formed on BT layer 110.2. Layer 110.3has vias 920 exposing the contact pads 350. Vias 920 form cavities inthe top surface of BT substrate 110. Silicon interposer contact pads 340will be inserted into these cavities to form a reliable mechanical andelectrical contact.

In one embodiment, each cavity 920 has a diameter Dcav=150 μm toaccommodate a 50 μm to 60 μm diameter Dc of the contact pads 340. Dc canbe calculated starting with the diameter Dv (FIG. 4) of via 330, bysubtracting double the thickness of the layers 410, 420, 430.1, 444,448. The depth Hcav of each cavity 920 (about equal to the thickness oflayer 110.3) is 50 μm for a 50 μm height Hd of contact pads 340 (Hcav ismeasured to the top surface of contact pads 350).

Cavities 920 are filled with solder paste 930. In one embodiment, thesolder paste is deposited to cover the BT substrate, and then is wipedoff by a squeegee blade to force the solder into cavities 920 and removeit from the top surface of BT layer 110.3.

The solder is chosen to have a high melting temperature to provide adesired solder hierarchy for subsequent solder attachment of dies 124and PCB 130. In some embodiments, the solder paste is a no-clean typeNC253 available from AIM of Montreal, Canada. This paste incorporatessolder flux but there is no need to clean the flux after the solderreflow.

No-flow underfill 940 (dielectric) is dispensed on BT substrate 110 atthe future site of interposer 120. In some embodiments, the underfill istype STAYCHIP™ 2078E available from Cookson Electronics, a companyhaving an office in Georgia, the United States of America. Thisunderfill performs both the underfill function and the solder fluxfunction. The underfill can be dispensed with a dispensing system oftype CAMELOT/SPEEDLINE 1818 available from Cookson Electronics.

Interposer wafer 120 is placed on BT substrate 110 (FIG. 10). Protrudingcontact pads 340 enter the cavities 920 and contact the solder 930 butdo not necessarily reach the metal 910 of contact pads 350. A uniformheight of contact pads 340 is not required for a good electricalcontact.

Underfill 940 spreads out under the interposer. In the embodiment shown,the bottom surface of silicon 140 does not reach the BT substrate.Underfill 940 helps insulate the silicon from solder 930. Therefore, itis unnecessary to form a dielectric layer on the bottom silicon surface.

The interposer placement can be performed with a placement tool of typeSFPLACE F4 available from Siemens corporation of Germany. The placementtool picks up the interposer from the top by a vacuum holder 1010schematically shown in FIG. 10. The vacuum pick-up flattens theinterposer if the interposer is warped. Dielectric 144 protects theinterposer from being damaged by the holder. Other placement tools, withvacuum and non-vacuum holders, known or to be invented, can alsopossibly be used.

The structure is heated to reflow the solder paste 930 and cure theunderfill 940. The solder wets the bottom and side surfaces of coppercontact pads 340. In one embodiment, the final value of the gap G1between the silicon 140 and the BT substrate 110 is 25 μm. The gapvalues of 5 to 10 μm and larger are believed to be appropriate toprovide sufficient electrical insulation if no dielectric is formed onthe bottom surface of silicon 140. The contact 340 portion inside thevias 920 is 25 μm high (C1=251 μm in FIG. 10). The value C1 is in therange from 10 μm to 45 μm in some embodiments.

Then vacuum holder 1010 releases the interposer.

In some embodiments, the vacuum holder releases the interposer beforethe solder reflow. The interposer stays in place due to a surfacetension between silicon 140 and the underfill 940. Multiple interposerscan be placed on BT substrate 110, and the solder reflow and underfillcuring can be performed in a single heating step for all theinterposers. A similar technique has previously been applied forflip-chip mounting of dies on a BT substrate, as described in M. Painaikand J. Hurtley, “Process Recommendations for Assembly of Flip Chipsusing No-flow Underfill”, Technical Bulletin, Cookson Semiconductor.

FIG. 11 illustrates another embodiment. The BT substrate 110 is similarto the BT substrate of FIGS. 9 and 10, but a metal layer 1110 is formedon the bottom and sidewalls of each cavity 920. Metal 1110 is believedto improve the strength and the electrical conductivity of the solderbond between contact pads 340 and contact pads 350. Metal layer 1110 canbe copper deposited on the BT substrate and patterned by lift-off orsome other process. In FIG. 11, metal 1110 extends out of cavities 920to the top surface of the BT layer 110.3 but does not provide anyinterconnects or other elements on the top surface of layer 110.3. Metal1110 is present only in the immediate vicinity of each cavity 920. Eachcontact 350 includes the portions of metal layers 910, 1110 on thebottom and sidewalls of the corresponding cavity 920. In otherembodiments, metal 1110 provides an additional level of interconnectsand/or a power or ground plane on layer 110.3.

In the BT embodiment described above, the BT layers 110.1, 110.2, 110.3are laminated on top of each other. Each layer 110.1, 110.2 is a solidsheet placed laminated on the structure in a solid form. In someembodiments, the top layer 110.3 is made from a material different fromthe material of layers 110.1, 110.2. For example, solder dam materialscan be used, such as photoimageable polyimide, Dupont VACREL 8100,Dupont Flexible PhotoImageable Coverlay (PIC) 1000 & 2000, Shipley(Dynachem) DynaMASK 5000, Shipley ConforMASK 2500, and possibly others.Some of the solder dam materials (e.g. polyimide) can be deposited in aliquid (possibly viscous) form and then cured.

FIG. 12 is similar to FIG. 11, but solder balls 1210 have been attachedto contact pads 360C. Solder balls 1210 eliminate the need for solderballs 370 (FIG. 3) on dies 124. The packaging substrate manufacturer canprovide solder balls 1210 to simplify the die 124 attachment for thesubstrate buyers. Solder 1210 can be attached to the interposer at anyfabrication stage. In one embodiment, solder 1210 is attached to pads360C before the interposer wafer is thinned, i.e. before the stage ofFIG. 8. The interposer wafer is mechanically stronger at this stage andits heat dissipating capability is higher, so the interposer handling iseasier.

Metal 1110 may be omitted (as in FIG. 10).

In some embodiments, solder 1210 has a lower melting temperature thansolder 930. Therefore, solder 930 is not melted during the attachment ofdies 124.

In the embodiment of FIG. 12, solder 1210 has the same or higher meltingtemperature than solder 930, but the melting temperature of solder 930is increased during the attachment of interposer 120 to BT substrate110. The melting temperature of solder 930 becomes higher than themelting temperature of solder 1210. The melting temperature of solder930 is increased because the copper from layer 1110 and/or layer 350dissolves in solder 930. In the embodiment of FIG. 12, copper 430.1 wasnot etched away as in FIG. 8, so copper 430.1 can also dissolve in thesolder. In some embodiments, solders 1210, 930 are initially the samesolder (i.e. the same material), which simplifies the wafer fabrication.For example, a eutectic solder Sn/Ag3.0/Cu0.5 (known as type LF128 fromAIM) can be used.

Metal contact pads 136C can be formed from a material other, thancopper. In some embodiments, interconnects 136 are made of copper, butcontact pads 136C are plated with a layer 1220 of nickel or gold. Layer1220 does not dissolve in solder 1210 and provides a barrier for thecopper diffusion from interconnects 136, so the melting temperature ofsolder 1210 does not change. In other embodiments, the meltingtemperature of solder 1210 changes during the attachment of theinterposer to substrate 110, but the melting temperature of solder 1210remains below the melting temperature of solder 930.

FIGS. 13-14 illustrate a possible manufacturing sequence with multipledie levels 124.1, 124.2, 124.3 attached to the packaging substrate. Thepackaging substrate is manufactured as in FIG. 12. The interposer viasare marked 330.0 (instead of 330 as in FIG. 12), the contact pads at thebottom of the interposer are marked 340.0, and the solder at the top ismarked 1210.0.

Each die 124.1 has one or more metalized through vias 330.1 formed inthe die's semiconductor substrate 140.1 (e.g. monocrystalline silicon).Each via 330.1 passes between the top and bottom surfaces of substrate140.1. Conductive paths are provided from contact pads at the top of thedie 124.1 to contact pads 340.1 at the bottom of the die through thevias 330.1. Contact pads 340.1 protrude out of the respective vias330.1. The dies 124.1 can be manufactured using the same techniques asdescribed above for interposer 120 (involving the wafer thinning toexpose the contact pads 340.1). Each die may have the same generalstructure as interposer 120 in FIG. 12. Of course, the circuitry in dies124.1 does not have to be identical to the interposer circuitry, anddifferent dies 124.1 may differ from each other. Also, contact pads340.1 may have smaller dimensions, and may be placed closer to eachother, as they do not have to meet the BT substrate dimensionrequirements. Pads 340.1 can be copper/nickel/gold structures as in FIG.12, or they can be made from other materials. The metal in vias 330.1 isinsulated from substrate 140.1 by a dielectric 410 (FIG. 12).

In some embodiments, dies 124.1 have devices (e.g. transistors, diodes,and others) manufactured at the top surface (active surface). Solderballs 1210.1 are attached to the contact pads on top of the dies,possibly before the wafer thinning operation exposing the contact pads340.1, as in FIG. 12.

Dies 124.2 may be similar to dies 124.1, but there is no solder on dies124.2. Dies 124.2 include metalized vias 330.2 in semiconductorsubstrates 140.2, and contact pads 340.2 protruding out of the vias. Theactive surface of dies 124.2 is the top surface in some embodiments.

The third level dies 124.3 are like dies 124 in FIG. 3. Their activesurface is the bottom surface. Solder 370 is attached to the bottomcontact pads.

The manufacturing sequence is shown in FIG. 14. Solder 1210.0 isattached to interposer 120, possibly before the interposer thinning(step 1410). Then the interposer is attached to BT substrate 110 asdescribed above (step 1420). During this step, the melting temperatureof solder 930 (FIG. 12) increases and becomes higher than the meltingtemperature of solder 1210.0. Solder 1210.0 may or may not be meltedduring this step. The melting of solder 1210.0 does not present aproblem because the dies 124.1 have not yet been attached to theinterposer.

In some embodiments, all of solders 120.0, 120.1, 930, 370 are initiallythe same material. In an illustrative example, the solders are eutectictype LF128 described above, with the initial melting temperature of 218°C. The melting temperature of solder 930 increases to about 230° C. instep 1420.

At step 1430, dies 124.1 are soldered to interposer 120 with solder1210.0, at a temperature of about 218° C. or higher, but below 230° C.not to melt the solder 930. The copper from contact pads 340.1 dissolvesin solder 1210.0 and increases its melting temperature to about 230° C.Solder 1210.1 may melt, but its melting temperature does not increasebecause the solder 1210.1 is not in contact with copper or othermaterial that could increase the solder melting temperature (the topsurface portions of the top contact pads of die 124.1 are made ofsuitable materials to ensure that the solder melting temperature doesnot increase).

At step 1440, dies 124.2 are attached to dies 124.1 with solder 1210.1.Solders 1210.0 and 930 do not melt. The melting temperature of solder1210.1 is increased to about 230° C. due to the diffusion of copper fromcontact pads 340.2.

At step 1450, dies 124.3 are flip-chip attached to dies 124.2 withsolder 370. Solders 930, 1210.0, 1210.2 do not melt. If desired, the topcontact pads on dies 124.2 may have copper to increase the meltingtemperature of solder 370. The higher melting temperature may bedesirable to prevent the solder melting during the attachment of BTsubstrate 110 to PCB 130 (FIG. 3). For example, the solder 134 used forthe PCB attachment may be the same material (LF128) as used for theprevious steps.

Many variations are possible. For example, any number of dies can beused at each level. Also, one or more dies 124.2 can be attacheddirectly to interposer 120, i.e. there may be three levels of dies overone interposer area but only two levels of dies over another interposerarea. Any number of die levels can be present in different interposerareas.

Other solder types and melting temperatures can be used, and materialsother than copper can be used to increase the melting temperatures.Different materials and contact pad structures can be used in differentdies. The semiconductor substrates can be different semiconductormaterials.

Varying the solder melting temperature to achieve a desired solderhierarchy is not limited to the interposer structures, but may be usedin other semiconductor packages, known or to be invented, with orwithout interposers.

In some embodiments, interposer 120 and/or dies 124.1 are provided withdeep cavities 920 at the top surface to increase the mechanical strengthof the solder attachment and provide a reliable electrical contact. SeeFIG. 15. The attachment of dies 124.1 to interposer 120 is performed bythe same techniques as the attachment of the interposer to BT substrate110. The attachment of dies 124.2 to dies 124.1 can also be performed inthis way.

As shown in FIG. 15, the top dielectric layer 144 in interposer 120 is athick layer, e.g. 50 μm thick. This can be a photoimageable materialsuch as described above for BT layer 110.3. Openings in top layer 144expose contact pads 136C. Contact pads 340.1 on die 124.1 protrude bysome distance, e.g. 50 μm, below the bottom surface of silicon substrate140.1 of die 124.1. The contact pads are inserted into the cavities inthe top surface of the interposer. These cavities are the openings intop layer 1-44 that expose the contact pads 136C.

Metal layer 1110 (e.g. gold or nickel) can be deposited on the sidewallsand bottom of the vias in top layer 144 to improve the electricalconnection and provide a barrier against copper 136 diffusion intosolder 1210.0. Alternatively, metal 1110 can be plated only on thebottom of the openings to provide a copper diffusion barrier.

In some embodiments, the same dimensions are obtained as for theattachment between the BT substrate and the interposer, i.e. the finalvalue of the gap between the silicon 140.1 and interposer 120 is 25 μm(gap values of 5 to 10 μm and larger are believed to be appropriate toprovide sufficient electrical insulation if no dielectric is formed onthe bottom surface of silicon 140.1); the contact 340.1 portion insidethe cavities in top layer 144 is 25 μm high (note dimension C1 in FIG.10). This value is in the range from 10 μm to 45 μm in some embodiments.Other dimensions can also be used.

Underfill (not shown) can be injected between the interposer and thedies 124.1 using known techniques.

In some embodiments, dies 124.1 are attached to interposer 120 beforethe interposer is thinned. See the aforementioned U.S. Pat. No.6,322,903. The attachment process can be the same as the process ofattaching the interposer to BT substrate 110. For example, in someembodiments, before the interposer is thinned, solder paste 1210.0 isplaced into the cavities on top of the interposer, then a no-fillunderfill is dispensed and a die or dies 124.1 placed on the interposer,then a heating step is performed. A copper diffusion barrier can beomitted. Copper 1110 and/or 136 on top of the interposer and copper430.1 from dies 124.1 dissolves in solder 1210.0 to increase the soldermelting temperature. Then interposer 120 is thinned and attached to BTsubstrate 110. Solder 1210.0 will not melt during the attachment ofinterposer 120 to BT substrate 110.

The invention is not limited to the embodiments described above. Forexample, non-eutectic solders can be used. The “melting temperature” isany temperature as high or higher than the solidus and but not higherthan the liquidus. As is known, the solidus is the highest temperatureat which 100% of solder is solid, i.e. the solder is just beginning tomelt. The liquidus is the lowest temperature at which 100% of the solderis liquid. For a eutectic solder, the solidus and the liquidus are thesame.

Also, in some embodiments, the cavities 920 (FIG. 9) extend through twoor more BT layers, for example, through layers 110.3 and 110.2. Contactpads 350 can thus be formed from the metal layer 910 located between theBT layers 110.1, 110.2. The layer 910 on BT layer 110.2 can be used forinterconnects, power or ground planes, or other elements as discussedabove. The invention is not limited to particular materials, dimensionsand processes. For example, anisotropic adhesive, conductive epoxy,and/or thermocompression can be used instead of solder. The invention isapplicable to non-silicon semiconductor interposers.

The interposer may include capacitors having a capacitance of 5.0 pF orhigher. For example, capacitance values of 10 pF, 100 pF, or higher havebeen used on circuit boards to decouple the power lines from the groundlines or for other purposes, and such capacitors can be manufactured inthe interposer. Resistors having resistance values of 10 Ω and higher(e.g. 50 Ω, 100 Ω, or 150 Ω) are used on circuit boards for linetermination and other purposes, and they can be manufactured in theinterposer. Inductors having inductance values of 100 nH or higher arecommonly used on circuit boards and can be manufactured in theinterposer. The invention is not limited to particular capacitance,resistance or inductance values. Other embodiments and variations arewithin the scope of the invention, as defined by the appended claims.

1. A manufacturing method comprising: (1) obtaining an interposercomprising: a semiconductor substrate; one or more first conductivecontact pads attachable to circuitry placed above the interposer; one ormore second conductive contact pads attachable to circuitry placed belowthe interposer; and one or more conductive paths passing through thesemiconductor substrate and connecting at least one of the first contactpads to at least one of the second contact pads; wherein each of thesecond contact pads protrudes out at a bottom surface of the interposer;(2) obtaining an intermediate integrated circuit packaging substratecomprising: a dielectric substrate or a plurality of dielectricsubstrates attached to each other; one or more first conductive contactpads attachable to circuitry above the intermediate substrate; one ormore second conductive contact pads attachable to circuitry below theintermediate substrate; one or more conductive paths each of whichconnects at least one first contact pad of the intermediate substrate toat least one second contact pad of the intermediate substrate; whereineach of the one or more first contact pads of the intermediate substrateis formed in a corresponding via in the top surface of the intermediatesubstrate, each via extending into at least one of the dielectricsubstrates; (3) inserting the protruding second contact pads of theinterposer into the corresponding vias of the intermediate substrate andattaching the second contact pads to the first contact pads of theintermediate substrate without melting of at least portions of thesecond contact pads of the interposer in the vias.
 2. The method ofclaim 1 wherein the operation (3) is performed without melting of anyportion of the second contact pads of the interposer.
 3. The method ofclaim 1 wherein the operation (3) comprises soldering the second contactpads of the interposer to the first contact pads of the intermediatesubstrate with solder which is not part of the second contact pads ofthe interposer.
 4. The method of claim 1 wherein after the operation (3)a spacing between the bottom surface of the semiconductor substrate andthe top surface of the intermediate substrate is at least 5 μm.
 5. Themethod of claim 1 wherein at least a portion of the bottom surface ofthe semiconductor substrate is not covered by any dielectric layer inthe interposer.
 6. The method of claim 1 wherein in the operation (3) atleast 10 μm of each protrusion is inside of the corresponding via. 7.The method of claim 1 wherein the intermediate substrate comprises saidplurality of the dielectric substrates.
 8. The method of claim 7 whereineach of the vias in the intermediate substrate passes through at leastone of the dielectric substrates.
 9. The method of claim 7 wherein theadjacent dielectric substrates are separated by conductive layers, andat least one of the conductive paths of the intermediate substratepasses through the conductive layers and through the dielectricsubstrates.
 10. The method of claim 9 wherein all of the dielectricsubstrates are made of the same material.
 11. The method of claim 1wherein the dielectric substrate or substrates are made of an organicmaterial.
 12. The method of claim 1 wherein the dielectric substrate orsubstrates are made of bis-maleimide triazine (BT).
 13. A manufacturingmethod comprising: (1) obtaining a first structure comprising: a firstsemiconductor substrate; one or more first conductive contact padsattachable to circuitry placed above the first structure; one or moresecond conductive contact pads attachable to circuitry placed below thefirst structure; and one or more conductive paths passing through thefirst semiconductor substrate and connecting at least one of the firstcontact pads to at least one of the second contact pads; wherein each ofthe second contact pads is provided by a conductor formed in acorresponding via in the first semiconductor substrate and protrudingdownward out of the via and out of the first structure at a bottomsurface of the first structure, the conductor providing a downwardprotrusion underneath the via at the bottom surface of the firststructure; (2) obtaining a second structure comprising: a secondsemiconductor substrate; a dielectric layer overlying the secondsemiconductor substrate; one or more first conductive contact padsattachable to circuitry above the second structure; wherein each of theone or more first contact pads of the second structure is formed in acorresponding via in the top surface of the second structure, each viaextending into the dielectric layer; (3) inserting the protrusionsformed by the conductors of the first structure into the correspondingvias of the second structure and attaching the protrusions to the firstcontact pads of the intermediate substrate in the vias in the secondstructure.
 14. The method of claim 13 wherein the second structurefurther comprises: one or more second conductive contact pads attachableto circuitry below the second structure; and one or more conductivepaths each of which passes through the second semiconductor substrateand connects at least one first contact pad of the second structure toat least one second contact pad of the second structure.
 15. The methodof claim 13 wherein the second structure is an interposer which is anintermediate integrated circuit packaging substrate.
 16. The method ofclaim 13 wherein after the operation (3) the bottom surface of the firstsemiconductor substrate is spaced from the top surface of the secondstructure.
 17. The method of claim 16 wherein after the operation (3) aspacing between the bottom surface of the first semiconductor substrateand the top surface of the second structure is at least 5 μm.
 18. Themethod of claim 13 wherein at least a portion of the bottom surface ofthe first semiconductor substrate is not covered by any dielectric layerin the first structure.
 19. The method of claim 13 wherein in theoperation (3) at least 10 μm of each protrusion is inside of thecorresponding via.
 20. A manufacturing method comprising: (1) obtaininga first structure comprising: a first semiconductor substrate; one ormore first conductive contact pads attachable to circuitry placed abovethe first structure; one or more second conductive contact padsattachable to circuitry placed below the first structure; and one ormore conductive paths passing through the first semiconductor substrateand connecting at least one of the first contact pads to at least one ofthe second contact pads; wherein each of the second contact padsprotrudes out at a bottom surface of the first structure; (2) obtaininga second structure comprising: a second semiconductor substrate; adielectric layer overlying the second semiconductor substrate; one ormore first conductive contact pads attachable to circuitry above thesecond structure; wherein each of the one or more first contact pads ofthe second structure is formed in a corresponding via in the top surfaceof the second structure, each via extending into the dielectric layer;(3) inserting the protruding second contact pads of the first structureinto the corresponding vias of the second structure and attaching thesecond contact pads to the first contact pads of the second structurewithout melting of at least portions of the second contact pads of thefirst structure in the vias.
 21. The method of claim 20 wherein thesecond structure further comprises: one or more second conductivecontact pads attachable to circuitry below the second structure; and oneor more conductive paths each of which passes through the secondsemiconductor substrate and connects at least one first contact pad ofthe second structure to at least one second contact pad of the secondstructure.
 22. The method of claim 20 wherein the second structure is aninterposer which is an intermediate integrated circuit packagingsubstrate.
 23. The method of claim 20 wherein the operation (3) isperformed without melting of any portion of the second contact pads ofthe first structure.
 24. The method of claim 20 wherein the operation(3) comprises soldering the second contact pads of the first structureto the first contact pads of the second structure with solder which isnot part of the second contact pads of the first structure.
 25. Themethod of claim 20 wherein after the operation (3) a spacing between thebottom surface of the first semiconductor substrate and the top surfaceof the second structure is at least 5 μm.
 26. The method of claim 20wherein at least a portion of the bottom surface of the firstsemiconductor substrate is not covered by any dielectric layer in thefirst structure.
 27. The method of claim 20 wherein in the operation (3)at least 10 μm of each protrusion is inside of the corresponding via.